how do i know when to charge the mouse and keyboard on my IMAC?

IMAC late 2015. El Capitan version 10.11.5. 1.6 GHZ. Intel i5. 8GB.

How do I know when to charge my rechargeable keyboard and mouse? How do I know when it is fully charged. Where do I find info on my IMAC so I don't have to ask about every little thing. The IMAC is new to me. Thanks